The present invention relates to an error correcting apparatus and, more particularly, to an error correcting apparatus which receives a signal subjected to a repetition processing for repeatedly transmitting a part of bits of an error-correction code train, and which restores the received signal to the original data train by subjecting the signal to a repetition regeneration processing and an error correcting decoding processing.
An error-correction coding technique is adopted so as to correct an error contained in received information or regenerated information and to restore it to the correct original information. Various codes such as a convolutional code and a turbo code are known as an error-correction code, and the error-correction coding technique is applied to various systems. In CDMA mobile communication, for example, an error-correction encoder 1 subjects information to be transmitted to an error-correction encoding processing, and a CDMA transmitter 2 subjects the code obtained with an error-correction code to a spread modulation processing and transmits it from an antenna, as shown in FIG. 12A. On the other hand, on the reception side, a soft decision error-correction decoder 4 subjects a soft decision data train obtained by the despreading operation and the RAKE combining operation of a CDMA receiver 3 to an error-correction processing, decodes the data and outputs the original transmitted information before the error-correction encoding processing, as shown in FIG. 12B. A soft decision data a 1-bit data represented by a plurality of bits depending upon the level.
FIG. 13 shows the structure of a CDMA transmitter in a mobile station. The error-correction encoder 1 subjects a data to be transmitted to an error-correction encoding processing and inputs it into a mapping portion 21. A control data generator 22 generates a control data such as a pilot PILOT and inputs it into the mapping portion 21. The mapping portion 21 outputs an error-correction code as an in-phase component data, and the control data as quadrature component respectively for quadrature modulation at a constant symbol rate. Spreaders 23a, 23b subject the in-phase (I) component and the quadrature (Q) component which are input from the mapping portion 21 to spreading modulation by using a predetermined spreading code, and input the spread data into DA converters 25a, 25b, respectively, via waveform shaping filters 24, 24b. A QPSK quadrature modulator 26 subjects an Ich signal, and a Qch signal output from each DA converter to QPSK quadrature modulation, and a radio transmitter 27 converts the frequency of a baseband signal output from the quadrature modulator 26 into a radio frequency (IFxe2x86x92RF), amplifies the frequency, and transmits the signal from an antenna.
FIG. 14 shows the structure of a CDMA receiver 3 for 1 channel in a CDMA receiving apparatus at a base station. A radio receiver 31 converts the frequency of a high-frequency signal received from an antenna into a frequency of a baseband signal (RFxe2x86x92IF). A QPSK quadrature detector 32 subjects the baseband signal to quadrature detection and outputs an in-phase (I) component data and a quadrature (Q) component data. In the quadrature detector 32, the reference numeral 32a denotes a receiving carrier generator, 32b a phase shifter for shifting the phase of a receiving carrier by xcfx80/2, and 32c and 32d multipliers for multiplying a baseband signal by a receiving carrier and outputting an I component signal and a Q component signal. Low-pass filters (LPF) 33a, 33b limit the band of an output signal, and AD converters 35a, 35b convert an I component signal and a Q component signal, respectively, into digital signals, and input them into a searcher 36 and each of the finger portions 37a1 to 37a4. 
When a direct sequence signal (DS signal) influenced by a multi-path is input into the searcher 36, the searcher 36 detects the multi-path by an autocorrelation operation using a matched filter (not shown), and inputs the data on the timing for starting the despreading operation and the data on the delay time adjustment in each path constituting the multi-path into the corresponding finger portions 37a1 to 37a4. A despreading/adjustment time adjuster 41 of each of the finger portions 37a1 to 37a4 subjects a direct wave or a delayed wave which reaches via a predetermined path to a dispreading processing by using the same code as the spreading code for the purpose of dump integration, thereafter subjects it to a delay processing in accordance with the path and outputs a pilot signal (reference signal) and an information signal. A phase compensator (channel estimation unit) 42 averages the voltages of the I components and the Q components of the pilot signals for a predetermined number of slots, and outputs channel estimation signals It, Qt. A synchronous detector 43 restores the phases of the despread information signals Ixe2x80x2, Qxe2x80x2 to the original phases on the basis of the phase difference xcex8 between the pilot signal contained in the received signal and a known pilot signal. That is, since the channel estimation signals It, Qt are the cos component and the sin component of the phase difference xcex8, the synchronous detector 43 demodulates (executes synchronous detection of) the received information signals (I, Q) by applying a phase rotation processing to the received information signals (Ixe2x80x2, Qxe2x80x2) by using the channel estimation signals It, Qt in accordance with the following formula:       (                            I                                      Q                      )    =            (                                                  I              t                                                          Q              t                                                                          -                              Q                t                                                                                        I                t                            ,                                          )        ⁢          xe2x80x83        ⁢          (                                                  I              xe2x80x2                                                                          Q              xe2x80x2                                          )      
A Rake combiner 37b combines the signals output from the finger portions 37a1 to 37a4, and outputs the combined signals to the soft decision error-correction decoder 4 (FIG. 12) as a soft decision data train.
FIG. 15 is an explanatory view of the frame format of an up signal transmitted from a mobile station to a base station. 1 frame is 10 msec and it is composed of 15 slots S0xcx9cS14. The data portion is mapped in an orthogonal I channel for QPSK quadrature modulation, and the portions other than the data portion are mapped in an orthogonal Q channel for QPSK quadrature modulation. The channel transmitting the data portion is called a DPDCH (Dedicated Physical Data Channel), and the channel transmitting the portions other than data is called a DPCCH (Dedicated Physical Control Channel). Each slot of the DPDCH (I channel) transmitting the data portion is composed of n bits, and n changes in accordance with a symbol rate.
FIG. 16A shows the relationship among the symbol rate (ksps), the number n of bits per slot, and the data length Nm (=15xc3x97n) per frame in the data channel DPDCH. The data channel DPDCH multiplexes and transmits the data in more than 1 transport channels. For example, the data channel DPDCH divides sound data into a sound data portion with a high degree of importance and a sound data portion with a low degree of importance, allocates a predetermined number of bits per frame to the respective sound data, multiplexes and transmits the data in different transport channels.
Each slot of the DPCCH (Q channel) for transmitting a control data is composed of 10 bits (see FIG. 15), and transmits a pilot PILOT, a transmission power control data TPC, a transport format combination indicator TFCI, and feedback information FBI at a constant symbol rate of 15 ksps. It is possible to change the number of bits of PILOT, TPC, TFCI, and FBI as occasion demands, as shown in FIG. 16B. PILOT is utilized when the reception side performs synchronous detection or measures a signal interference ratio SIR, TPC is utilized for the control of a transmission power, TFCI indicates the symbol rate or the number of bits per frame of data, the number of bits of data which increases by repetition, etc., and FBI is used to control the diversity transmission in the base station.
FIGS. 17A and 17B are explanatory views of the frame format and the structure of the slots of a down signal transmitted from a base station to a mobile station. 1 frame is 10 msec, and it consists of 15 slots S0xcx9cS14. Each slot is constituted by k bits, wherein k varies in accordance with the symbol rate. Each slot transmits a first data portion DATA1, a second data portion DATA2, a pilot PILOT, a transmission power control data TPC, and a transport format combination indicator TFCI. The number of bits of PILOT, TPC, TFCI vary in accordance with the symbol rate, as shown in FIG. 17B, and even if the symbol rate is the same, the number of bits varies as occasion demands. The data in each slot is alternately distributed into the I channel and the Q channel for QPSK quadrature modulation, and after it is subjected to spread modulation and quadrature modulation, the data with the frequency converted is transmitted to the mobile station.
As shown in FIGS. 16A and 17B, the maximum number of bits (data length Nm) per frame of data which can be transmitted depends on the system of connection between stations such as a symbol rate. On the other hand, since the error-correction code length Nc which is obtained by applying an error-correction coding processing to information to be transmitted varies in accordance with the length NI of the information to be transmitted, the relationship Nc=Nm does not always hold. For this reason, if Nc less than Nm, as shown in FIG. 18, the period S during which no data is transmitted generates. In addition, in the case of multiplexing information to be transmitted in a plurality of transport channels, the sum of the lengths of data in the plurality of transport channels does not usually coincide with the maximum data length Nm, and the period S during which no data are transmitted generates.
In order to effectively utilize the period S during which no data is transmitted, a repetition processing for repeatedly transmitting a part of bits of the error-correction code train is applied so as to make the total length of the information to be transmitted coincide with the maximum data length Nm. According to this repetition processing, the transmission energy per bit increases, which leads to an increase in the error-correction ability of the reception side.
FIGS. 19A and 19B show the structures of a transmission system and a reception system, respectively, provided with a repetition function. In the transmission system shown in FIG. 19A, a repetition processor 5 is provided between the error-correction encoder 1 and the CDMA transmitter 2. In the reception system shown in FIG. 19B, a repetition regenerator 6 is provided between the CDMA receiver 3 and the soft decision error-correction decoder 4.
In the transmission system, the error-correction encoder 1 subjects information to be transmitted to an error-correction coding processing and produces error-correction codes, and the repetition processor 5 subjects the error-correction codes to a repetition processing in accordance with a repetition algorithm. Due to the repetition processing, a part of the error-correction code train appears a plurality of times in the train subjected to the repetition processing, as shown in FIG. 20. In the example shown in FIG. 20, second, fifth, eighth, eleventh, fourteenth, . . . bits are repeated. The CDMA transmitter 2 applies the spread modulation to the data which has been subjected to the repetition processing, and transmits the data.
In the reception system, the CDMA receiver 3 demodulates a received signal, and inputs a soft decision data train A (see FIG. 21) having a predetermined bit width which is obtained by demodulation to the repetition regenerator 6. The repetition regenerator 6 executes a repetition algorithm so as to identify the bits (second, fifth, eighth, eleventh, fourteenth, . . . ) which are repeatedly transmitted due to the repetition processing, adds the soft decision data which corresponds to the bits, and converts the result into a soft decision data train B which corresponds to the original error-correction code train. The soft decision data is constituted by sign bits and soft decision bits. Thenceforth, the soft decision error-correction decoder 4 executes an error-correction decoding processing by using the soft decision data train B which is output from the repetition regenerator 6, and restores the data to the original data train before the error-correction coding processing.
Each of the repetition processor 5 and the repetition regenerator 6 executes the following repetition algorithm. The parameters in the algorithm are as follows:
(1) N: number of bits of the data before the repetition processing
(2) xcex94N: number of bits repeated due to the repetition
(3) (N+xcex94N): number of bits of the data after the repetition processing
(4) e: parameter of the error which is updated in the algorithm (whether bits should be repeated or not is determined by judging the error e)
(5) e-ini: parameter used for determining the initial value of the error e
(6) e-plus: constant to be added to the error e when the error e is not more than 0 and predetermined bits are repeated (e-plus=axc2x7N)
(7) e-minus: constant used for updating the error e (e-minus=axc2x7AN)
(8) a: parameter used for determining e-plus or e-minus (e.g., a=2) In other words, execution of the repetition algorithm is enabled by giving the above 5 parameters N, xcex94N, e-ini, e-plus, and e-minus, thereby enabling the decision of the repetition bits.
Repetition Algorithm
e=e-ini
m=1 (m is an interested bit)
do while mxe2x89xa6N (execute the following as long as this relationship holds)
e=exe2x88x92e-minus (e-minus=axc2x7|xcex94N|)
do while exe2x89xa60 (execute the following as long as this relationship holds)
repeat bit xm (repeat m-th bit Xm)
e=e+e-plus (update e:e-plus=axc2x7N)
end do
m=m+1 (increment the interested bit)
end do
As a concrete example of the repetition algorithm, an algorithm in which the number of multiplexed transport channels is 1, the data length N of the error-correction codes is 216 bits, and the data length (N+xcex94N) after the repetition processing is 240 bits is shown in FIGS. 22 to 26. The parameters used in the repetition are N=216, xcex94N=24, e-ini=1, e-plus=432 and-e minus=48. As is clear from FIGS. 22 to 26, when the sign of the error e is minus as a result of the subtraction of 48, the bit is repeated (RIS=xe2x80x9c1xe2x80x9d). In FIGS. 22 to 26, 1st, 10-th, 19-th, 28-th . . . 208-th bits are repeated.
As described above, the repetition regenerator 6 (FIG. 19) adds the bits which are repeatedly transmitted due to the repetition and generates the soft decision data train which corresponds to the original error-correction code train. As is clear from FIG. 21, the bit width of the soft decision data train increases by 1 bit due to the addition. In the example shown in FIG. 21, the number of soft decision bits is 5 including the sign bit before the repetition regeneration. However, after the repetition regeneration, the number of soft decision bits is 6 including the sign bit. In this manner, when the bit width of the soft decision data which is input into the error-correction decoder 4 increases, the circuit scale of the error-correction decoder 4 which uses a convolutional code or a turbo code inconveniently enlarges.
As a method for preventing the circuit scale of the error-correction decoder 4 from being enlarged, there is known a method of inputting the soft decision data which is output from the repetition regenerator 6 into the error-correction decoder 4 after the bit width of the data is reduced. In this method, (the sign bit+upper m bits) or (the sign bit+lower m bits) are cut off from the soft decision data of (the sign bit+(m+1)) bits which is output from the repetition regenerator 6, and input into the error-correction decoder 4. In the example shown in FIG. 21, (1) (the sign bit+upper 4 bits) of the soft decision data train B are input into the error-correction decoder 4, or (2) (the sign bit+lower 4 bits) of the soft decision data train B are input into the error-correction decoder 4.
There is, however, a problem in reducing the bit width by cutting off a part of the data at the same bit position irrespective of the repetition rate Rr (=(N+xcex94N)/N). That is, the amount of deterioration in the error-correction ratio in the error-correction decoder 4 increases. FIG. 27 shows the relationship between the repetition rate and the amount of characteristic deterioration. In the case (1) of selecting the upper m bits, the smaller the repetition rate Rr is, the larger is the deterioration of the error-correction characteristic. In contrast, in the case (2) of selecting the lower m bits, the larger the repetition rate Rr is, the larger is the deterioration of the error-correction characteristic.
The reason is as follows. FIG. 28 shows the distribution of the signal having a predetermined size in correspondence with the repetition rate Rr. The larger the repetition rate Rr is, the larger is the range in which the most significant upper bit is valid (see the hatched area). In other words, as the repetition rate Rr becomes larger, it becomes more difficult to exactly represent the soft decision data output from the repetition regenerator by the lower m bits, while it is possible to represent them with accuracy by the upper m bits. As a result, when the repetition rate Rr becomes larger, the range where the most significant upper bit is valid enlarges, while when the repetition rate Rr becomes smaller, the range where the most significant upper bit is invalid enlarges, as shown in the tendency in FIG. 27.
Accordingly, it is an object of the present invention to eliminate the above-described problems in the related art, and to reduce the bit width of a soft decision data after repetition regeneration without deteriorating the error-correction characteristic due to the reduction of the bit width.
To achieve this object, in a first aspect of the present invention, there is provided an error correcting apparatus comprising a repetition regenerator, a repetition rate calculator, a soft decision data cut-off position decision unit, and a soft decision data cutting means. The repetition regenerator obtains the positions of the bits which are repeatedly transmitted due to a repetition processing, adds the soft decision data corresponding to the positions of the bits and generates a soft decision data train which corresponds to the original error-correction code train. The repetition rate calculator calculates the repetition rate of a received signal which is subjected to the repetition processing. More specifically, the repetition rate calculator calculates the repetition rate Rr=(N+xcex94N)/N on the basis of the number N of bits of the data which is notified of from the communication party before a repetition processing and the number xcex94N of bits which are repeated due to the repetition. The soft decision data cut-off position decision unit decides the position at which a part of the soft decision data which is to be input into the soft decision error-correction decoder is cut off from the soft decision data which is produced by the repetition regenerator, on the basis of the repetition rate. The soft decision data cutting means cuts off the part of the soft decision data on the basis of the decided cut-off position and inputs it into the soft decision error-correction decoder.
The amount of characteristic deterioration increases or decreases depending upon the position at which the part of the soft decision data is cut off, but this tendency reverses at a predetermined repetition rate RTH. More specifically, if the actual repetition rate Rr is not more than RTH (Rrxe2x89xa6RTH), the amount of characteristic deterioration is smaller when the part of soft decision data is cut off at a first cut-off position which is the lower bit portion, but if Rr exceeds RTH (Rr greater than RTH), the amount of characteristic deterioration is smaller when the part of soft decision data is cut off at a second cut-off position which is the upper bit portion. According to an error correcting apparatus provided in a first aspect of the present invention, it is possible to suppress the deterioration of the characteristic by switching the position at which the part of the soft decision data is cut off on the basis of the repetition rate.
In a second aspect of the present invention, there is provided an error correcting apparatus comprising a repetition regenerator, an average value calculator, a soft decision data cut-off position decision unit, and a soft decision data cutting means. The repetition regenerator obtains the positions of the bits which are repeatedly transmitted due to a repetition processing, adds the soft decision data corresponding to the positions of the bits, and generates a soft decision data train which corresponds to the original error-correction code train. The average value calculator calculates the average value of the soft decision data which is output from the repetition regenerator. The soft decision data cut-off position decision unit decides the position at which the part of the soft decision data which is to be input into the soft decision error-correction decoder is cut off from the soft decision data which is output from the repetition regenerator, on the basis of the average value. The soft decision data cutting means cuts off the part of the soft decision data at the decided cut-off position and inputs it into the soft decision error-correction decoder.
The amount of characteristic deterioration increases or decreases depending upon the position at which the part of the soft decision data is cut off, but this tendency reverses at a predetermined average value VTH of the soft decision data. More specifically, if the actual average value Va of the soft decision data is not more than VTH (Vaxe2x89xa6VTH), the amount of characteristic deterioration is smaller when the part of soft decision data is cut off at a first cut-off position which is the lower bit portion, but if Va exceeds VTH (Va greater than VTH), the amount of characteristic deterioration is smaller when the part of soft decision data is cut off at a second cut-off position which is the upper bit portion. According to an error correcting apparatus provided in a second aspect of the present invention, it is possible to suppress the deterioration of the characteristic by switching the position at which the part of the soft decision data is cut off on the basis of the average value of the soft decision data which is output from the repetition regenerator.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings.